L-theanine is an amino acid found in tea leaves. Previous studies of dietary supplements of L-theanine have suggested a therapeutic benefit in reducing stress, promoting relaxation, and improving the quality of sleep.
Akiko Higashiyama, from University of Shiga Prefecture (Japan), and colleagues have found that daily L-theanine supplementation at 200 mg/day helps people with anxiety to focus on their daily activities. The researchers recruited 18 healthy University students and assessed their anxiety levels using a standardised rating scale. Student with high anxiety were put in one group, while students with minimal anxiety were put in another group. Both groups received water or water plus 200 mg of L-theanine per 100 ml of water. 
The test was performed repeatedly and assessments performed between 15 and 60 minutes after ingestion. Results showed that highly anxious students receiving the L-theanine displayed a slowing of their heart rate, improved attentional performance, and better reaction times, compared to members of the same group receiving placebo. No significant benefits were observed in the student with minimal anxiety.
Submitting that: “The results demonstrate the significant enhanced activity of alpha bands, descending heart rate, elevated visual attentional performance, and improved reaction time response among high anxiety propensity subjects compared to a placebo,” the team concludes that:
“Results evidently demonstrated that L-theanine clearly has a pronounced effect on attention performance and reaction time response in normal healthy subjects prone to have high anxiety.”
Comment: The use of L-theanine as a supplement has been explored for a number of years and may explain some of the benefits associated with tea ingestion and mood benefit. The GABA receptors in the brain are beneficially altered by the ingestion of L-theanine and GABA, producing relatively fast reductions in anxiety, often as quickly as in the first 1 hour after ingestion.
 Akiko Higashiyama, Hla Hla Htay, Makoto Ozeki, Lekh R. Juneja, Mahendra P. Kapoor Effects of l-theanine on attention and reaction time response. Journal of Functional Foods, Volume 3, Issue 3, July 2011, Pages 171-178 View Abstract
 Abdou AM, Higashiguchi S, Horie K, Kim M, Hatta H, Yokogoshi H. Relaxation and immunity enhancement effects of gamma-aminobutyric acid (GABA) administration in humans. Biofactors. 2006;26(3):201-8.